Credit: Kevin Lamarque/Reuters

ABC News' Jeffrey Cook reports:

Edward Markey of Massachusetts was sworn in today as the newest member of the U.S. Senate, filling the seat previously occupied by Secretary of State John Kerry.

Markey, a Democrat, served in the House of Representatives for 37 years and defeated Republican Gabriel Gomez in a June special election after President Obama nominated Kerry to become the nation's chief diplomat.

Markey took his oath of office Tuesday in the Senate chamber of the U.S. Capitol building and will fill out the remaining year and a half of Kerry's term.

First elected to the House in 1976, Markey has established himself as one of the Democratic Party's leaders regarding energy issues. He was the ranking member of the National Resources Committee and ranked eighth in seniority in the House when he stepped down for the Senate seat.

Late in the race for the June special election, President Obama and Vice President Joe Biden joined Markey on his campaign.

William "Mo" Cowan served as the interim senator after Kerry's exit.

Cowan, former chief of staff to Massachusetts Democratic Gov. Deval Patrick, tweeted on Monday, "Been an honor to serve MA as Senator."

Cowan accompanied Markey, along with senior Sen. Elizabeth Warren, while Biden delivered the oath of office.

"John Kerry served here for 28 years, and for me to follow in his footsteps is a great honor," Markey said after the ceremony.

Markey's first day in the Senate came as Democrats and Republicans reached a tentative deal to avoid the use of the so-called "nuclear option" to limit Republicans' use of the filibuster against President Obama's executive nominees.

"The Senate has to be the institution that does work to ensure that the American people are being served," Markey said. "The House of Representatives right now is paralyzed. Hopefully a deal is reached today that begins to break the gridlock here in the Senate."